The invention relates to a current source circuit and, more particularly, to a current source circuit adaptable for use with an analog-to-digital converter.
One known analog-to-digital (hereinafter referred to as A-D) converters such as that disclosed in U.S. Pat. No. 3,316,547 compares an analog input current with a reference current through an integration circuit. Another type converter compares an analog input current with combinations of a plurality of weighted reference currents. In such an A-D converter, the reference current is obtained by converting to a current a voltage applied from an exterior stable reference voltage source. Particularly in the latter case, the reference voltage, or the reference divided voltage, is adjusted at the initial stage to set up a reference current value for the purpose of scale adjusting.
An A-D converter of the integration type will be described with reference to FIG. 1. An integrator 10 is comprised of an operational amplifier 3 and an integration capacitor Co connected between the output terminal and one of the input terminals of the operational amplifiers 3. A reference current source 1 feeds a reference current I.sub.ref to the integrator 10 through a contact a of a switch So when the switch So is turned to the contact a. A second source 2 feeds an unknown current Ix corresponding to an unknown voltage Vx to the integrator 10 through a contact b of the switch So when the switch is turned to the contact b. In current source, an unknown voltage Vxo is converted into the unknown current Ix and in current source 1 the reference voltage V.sub.ref is converted into the reference current I.sub.ref. In this type converter, the unknown current Ix is integrated by the integrator 10 from an initial value over a given period of time. Then, over the following period, the reference current I.sub.ref is inversely integrated until the integration value reaches the initial value. In this manner, the current Ix is made to correspond to the "following period" and the result of the integration is displayed digitally. This type of A-D conversion is discussed fully in the above-mentioned U.S. patent, and therefore no further explanation need be given here. Before the conversion operation, the reference current I.sub.ref is adjusted by adjusting the reference voltage V.sub.ref. In this case, the reference current I.sub.ref is generally made to correspond to the full scale of the converter, i.e. the maximum expected value of current Ix (max). Such a scale adjustment is effected during device set-up. However, drift may occur in the scale adjustment over a relatively long period of time because it is affected by its ambient temperature change and the change of the components' characteristics for example, due to the aging. The error in the scale adjustment is due primarily to two factors, an error due to the variation of the external reference voltage value V.sub.ref per se and an error due to the characteristic variation of the A-D converter per se. The latter error is generally called a gain error. Particularly in C-MOS monolithic A-D converters, the gain error is relatively large. This has necessitated the development of a circuit for automatically correcting the gain error. The gain error arises from the fact, for example, that the potential at the input terminal of the amplifier 3 in FIG. 1 changes in the course of the use of the converter so that offset value appears between the terminal c and the terminal d which varies. When the voltage or current from a temperature sensor using a thermistor or the like is subjected to an A-D conversion, what is detected is the ratio between the input and output voltages of the sensor and, therefore, in its application, the voltage applied to the sensor is essentially used as the reference voltage. For this, the variation of the reference voltage does not cause scale error. In this case, only the gain error causes the scale error. Thus automatic correction of the gain error has been required.